during the six or seven years preceding it.

It is impossible in the space at disposal to treat of this
development even with the meagre amount of detail that has been
possible while covering the 'settling down' period from 1911 to
1914, and it is proposed, therefore, to indicate the improvements
by sketching briefly the more noticeable difference in various
respects between the average machine of 1914 and a similar
machine of 1918.

In the first place, it was soon found that it was possible to
obtain greater efficiency and, in particular, higher speeds,
from tractor machines than from pusher machines with the air
screw behind the main planes. This was for a variety of reasons
connected with the efficiency of propellers and the possibility
of reducing resistance to a greater extent in tractor machines
by using a 'stream-line' fuselage (or body) to connect the main
planes with the tail. Full advantage of this could not be
taken, however, owing to the difficulty of fixing a machine-gun
in a forward direction owing to the presence of the propeller.
This was finally overcome by an ingenious device (known as an
'Interrupter gear') which allowed the gun to fire only when
none of the propeller blades was passing in front of the muzzle.
The monoplane gradually fell into desuetude, mainly owing to the
difficulty of making that type adequately strong without it
becoming prohibitively heavy, and also because of its high
landing speed and general lack of manoeuvrability. The triplane
was also little used except in one or two instances, and,
practically speaking, every machine was of the biplane tractor
type.